1. If you increase the gap between those two terrain meshes, it might get rid of the flickering.
2. You need 3D modelling programs like Sketchup or 3DS Max, probably Blender can work too.
3. Either you paint the grid on to the texture itself, or then you make the painted lines with wall tool. Use only two nodes in the wall editor, and bring the wall object very close to the road surface. Then it looks like it is painted there.
4. You make the tarmac same way as the racing road as well, like you said use open circuit and carefully try to align the ends of that road to the racing circuit.